In the above given description of a joint-prosthesis is indicated that the applied ball may in no case be completely enclosed in the cup-shaped element. For that reason the sphere-sector surface-shaped part of the contact surface has been limited to at the most the surface of a half ball.
In the European patent application 0 053 794 a cup-shaped element is described, for use in a joint prosthesis as indicated above, whereby in the upper-inner edge of the cup-shaped element a recess is present for taking up a ring. The cup-shaped element is manufactured from a ceramic material while the ring is made of a plastic deformable material. The ring serves to protect both the ball and the cup of said prosthesis in case of occurrence of sub-luxation.
In some occasions however, depending on the condition of the patient, a certain hampering of luxation is necessary allthough luxation should remain possible in order to prevent damage to the bone material in which the cup is fixed using the usual cementing procedures or with mechanical means.